Originally Posted by AaronWalkhouse

So, in order to impersonate another user on a typical broadband connection you
need three things, the right MAC address, physical access to the same line and
the opportunity to use both of those quickly enough to fool the ISP and any
potential investigators. The odds are strongly against it and the risk of being
caught in the attempt is high. It could also be done if the victim has an unsecured
wireless router but that circumstance also gives said victim a plausible defence.